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A counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.

In a charged transition metal complex, a simple (i.e. non-coordinated) ionic species accompanying the complex is termed the counterion.

Surfactants like cetyl trimethylammonium bromide form micelles in solution. The bromine counterions are retained around the micelles even in the presence of potassium chloride. Selective counterion condensation is observed with cetyl trimethylammonium chloride when the condensed chlorine ions are replaced by bromine on addition of potassium bromide [1]